The occurrence of local electric discharges, so-called partial discharges, in the insulation system for electric installation components or electric high-voltage apparatus, such as, for example, transformers, generators, cables, etc., constitute an important parameter for estimating the quality, condition and expected service life of the installation component. Normally, therefore, the acceptance tests for components for an electric high-voltage installation comprise a testing with respect to the occurrence of partial discharges under well defined conditions, for example according to IEC Publ. 270 "Partial Discharge Measurements". In addition thereto, various methods and devices have been developed for testing at the installation site for the purpose of assessing, after a certain time of operation, the condition of components and hence, for example, creating a more reliable basis for a decision on preventive maintenance.
One problem when measuring partial discharges is to distinguish measurement signals originating from partial discharges in the test object from disturbances originating from partial discharges in surrounding installation components or in the testing equipment, or which have some other origin but a frequency and amplitude spectrum which resembles the spectrum of the partial discharges. Such disturbances may be passed to the test object through the connection leads, or conductors, by which it is galvanically connected to the surroundings but may also, in the relevant frequency ranges, be captured by the measuring device through electromagnetic radiation from the surroundings.
A partial discharge in an installation component gives rise to current pulses through the conductors by which the component is connected to the surroundings and these current pulses may be sensed either by measuring the voltage caused thereby when they pass through a measuring impedance connected to one of the conductors, or by a direct or indirect current measurement. During voltage measurement, the measuring equipment comprises a coupling capacitor, which must be free from partial discharges under the conditions under which the measurement is carried out, and the test object, the coupling capacitor and the measuring impedance are galvanically interconnected, in a manner known per se, and connected to the surrounding installation components and to the test equipment, respectively.
When sensing the partial discharge through current measurement, sensors are required which are sensitive to the magnetic field associated with the current pulse. The measurement signal obtained from such sensors may be kept galvanically separated from the test object and the high-voltage circuit connected thereto, in which case, among other things, problems such as the effect of loops in the ground circuits is eliminated. Current-measuring sensors eliminate the need of coupling capacitors and measuring impedances and may be designed with very small dimensions, which makes it simple to arrange them at installation components of varying dimensions and shapes.
The unpublished international patent application PCT/SE96/01126 describes a device for sensing electric discharges in a test object with at least two electric connection leads, comprising evaluation equipment and two sensors, each one for direction-sensitive sensing of current pulses through a respective one of the connection leads. The sensors, for example in the form of Rogowski coils, sense a magnetic field, generated by the current pulses, and the polarity of this magnetic field, and produce, in dependence on these current pulses and their direction, one sensor signal each to the evaluation equipment. This equipment generates, in dependence on received sensor signals, an indication signal, indicating an electric discharge in the test object, when the sensor equipment senses current pulses which essentially simultaneously flow either in a direction out from the test object through all the connection leads or in a direction into the test object through all the connection leads. The evaluation equipment comprises means which form a sum signal in dependence on a sum of sensor signals originating from sensors associated with mutually separated connection leads, and a difference signal in dependence on a difference of sensor signals originating from sensors associated with mutually separated connection leads. A comparing member comprised in the evaluation equipment forms a comparison signal in dependence on the sum signal and the difference signal, for example as a quotient between these signals, whereby the indication signal is formed in dependence on the comparison signal.
The device comprises a filter unit, associated with each sensor and placed in the vicinity thereof, for tuning the natural frequency of the sensor, with a tuning circuit in the form of a resistor connected in parallel with a capacitor, and a bandpass filter for tuning to the series-resonance frequency of the test object.
Because of the physical distance between two sensors, placed on different sides of the connection leads of the test object, when an external disturbance passes through the connection leads of the test object, the sensors will generate their respective sensor signals at different times, that is, the physical distance between the sensors corresponds to a time distance between the sensor signals which emanate from the disturbance. This time distance may under certain circumstances, especially in test objects which because of their shapes only allow a location of sensors with a large mutual physical distance, by the method described in the patent specification involving addition and summation of sensor signals, lead to reduced reliability in the detection of external disturbances as opposed to internal discharges.
It has also been found that, in certain embodiments, the sensors, when they are subjected to a compressive force when being mounted on the test object, may exhibit an inductance which deviates from its nominal value. This fact, as well as, of course, manufacturing tolerances for the sensor, leads to the tuning circuit causing a mistuning of the sensor. This results in a deteriorated signal/noise ratio in the signal which is to be further processed in the evaluation equipment, which in turn requires adjustment of the tuning of the sensor by means of adjusting members provided for this purpose.